Transcript CEN5515 - Washington University in St. Louis

Lecture Note on Transmission Basic
Direct Connections in Communication Networks
• Building Blocks
– links: coax cable, optical fiber...
– nodes: general-purpose workstations...
• Connectivity Types
– point-to-point
– multiple access
Indirect Connections in Communication Networks
• Connectivity Types
– switched networks
=> switches
– inter-networks
=> routers
Connectivity Examples
• Internet:
– Best-effort
(no performance guarantees)
– Packet-by-packet
• Point-to-point link:
–
–
–
–
Always-connected
Fixed bandwidth
Fixed delay
Zero-jitter
Delays in Networks
• Propagation time or propagation delay, tprop
– Time required for a signal or waveform to propagate (or move) from one
point to another point. It is defined as d/v where d is the distance between
the two points and v is the velocity of the signal.
– Electromagnetic signal sent through space from the Sun to the Earth.
• Velocity is the speed of light, 3 × 108 meters per second.
• Say distance is 148.8 million kilometers.
• Propagation time is (148.8 /3) × 109 / 108 = 496 seconds.
•
Transmission time, ttrans
– Links or channels for digital transmission are defined in bits/second or
bps. This is called the Data Rate, R, and is the rate at which the
transmitter can push bits onto the transmission line. Transmission time is
the time it takes to send some number, say B bits, or B/R.
– What is the transmission time for a packet of 1500 bytes on a 10 Megabit
link.
• 1500 × 8 / 107 = 1.2 milliseconds.
• Queuing time or queuing delay
– Any time related to processing or waiting of packets in buffers.
Representation of Information
• Block Oriented Information: specific length blocks
– Text files
– Fax documents
• Example: A4 Page
• 200 pixels per inch (horizontal) by 100 pixels per inch (vertical) requires 256
kilobytes prior to compression (a pixel is black or white). Compression
reduces by a factor of 8 to 16.
– JPEG color images
• 8 x 10 inch photo uncompressed is about 38.4 Mbytes before compression
and about 1.2 – 8 Mbytes after compression.
– Note that each of 3 colors (RGB) requires one byte to represent the value
• Stream Oriented Information: continuous stream of data
–
–
–
–
Voice PCM: 4 kilohertz voice is 64 kbps, standard voice channel
Voice ADPCM, compression technique reduces rate to 16-32 kbps
Audio MPEG audio MP3 compression
Video MPEG 2
• 1920 x 1080 pixel frames at 30 frames /second requires about 1.5 Gbps
(includes 20bits/pixel and vertical/horizontal blanking and embedded audio)
uncompressed is reduced to 19-38 Mbps compressed
A4 Fax Document Standard
1/16 m2, 5 grams / page typically
210 millimeters x 297 millimeters
(slightly bigger than 8 1/2 by 11)
Data Compression Techniques
• Why data compression: reduce amount of storage space required
to represent the document
– Compression ratio: size of uncompressed / size of compressed
– Lossless data compression
• Example: run length encodings
– Sequences of likely symbols encoded into runs
– 25 0’s follows by a 1 is encoded as a fixed m-bit code word
– Example: encode 000000000000100001001 using a 3 bit code
– Lossy data compression
•
•
•
•
Compressed file is an approximation of the original data
Better compression ratio is achieved.
For example, MP3 can achieve a factor of 14 reduction.
Example: unencoded MP3 requires about 1.4 Mbits/sec (2 channels, 16 bits
per sample, at a rate of 44100 samples per second). Thus, 1 hour is about
317 MB (about ½ of an audio CD and 1/14 of common DVDs) Through MP3
compression, we can compress to say 128 kbits /second.
Analog Information
• Based on a wave that varies continuously with time
• Example
– Sound
– Electromagnetic radiation
• Periodic signal repeats over time
• Representation of a analog signals
– Amplitude versus time
– Frequency representation
– Representation as sums of sines and cosines
Frequency Spectrum
• Frequency: cycles per second (Hertz) of a sinusoidal signal
• Bandwidth: usable range of frequencies high – low: f2 – f1
• Typical telephone voice channel is 4k Hz
Some approximate frequencies
300 Hz – 20,000 Hz
human voice / sound
50 kHz
navigation (ships, submarines, etc)
1 MHz
AM radio (20 k Hz channels)
10 MHz
CB, short wave
100 MHz
FM radio, TV
1 GHz
UHF TV, mobile telephony
10 GHz
amateur satellite
100 GHz
upper microwave – many uses (up to about 300 GHz)
10 T Hz = 1013 Hz Infrared, 1015 Hz Visible light, 1018 X-rays, . . . .
Bandwidth Versus Channel Rate
• Shannon's theorem: maximum bit rate for noisy channel
C = Bandwidth * log2 (1 + S/N)
• 4 kHz voice channel equates to 56 or 64 k bits / second.
– This is achievable by modern modems
• Some sample bit rates
Radio LAN in the 2.4GHz band: 2 Mbps
Fast Ethernet: 100 Mbps, Gigabit Ethernet,
Optical fiber transmission: 2.4 – 9.6 Gbps over one wavelength
Multiplexing
• Multiplexing is fundamentally sharing some amount of
bandwidth by many different users, channels, etc.
– FDM: Frequency Division Multiplexing: dividing up the frequency
spectrum into multiple channels, each channel dedicated to a particular
use
– TDM: Time Division Multiplexing: using the entire frequency bandwidth
allotted but uses “logical slots” to transmit different channels at specific
points of relative time within a logical frame that is transmitted.
• A digital telephone speech signal (corresponding to a 4 kHz
channel) is obtained as follows
– Sample the signal at twice the maximum frequency or 8000 times a
second
– Quantize the sampled value ( the amplitude of the signal) to 7 bits or 128
different value.
– This gives a bit rate of 56 kbits/second
T-1 Carrier System
• T-1 was developed to carry digitally multiplexed channels
between central offices using TDM.
• T-1 multiplexed 24 voice channels in a T-1 frame as follows
Channel 1
Channel 2
. ..
...
Channel 24
7 bit data,
1 bit control
1 bit framing
Sampling rate: 1 frame is sent every 125 µsec (8000 frames /sec)
8 bits / channel x 24 channels + 1 bit = 193 bits per frame.
Therefore, 193 bits / 125 µsec = 1.544 Mbps
This standard signaling rate became known as DS-1 (digital signal 1)
DS Hierarchy
• DS-1: basic block, 1.544 Mbits/sec
• DS-2: 6.312 Mbits/sec (4 DS-1 or 96 voice, plus some control)
• DS-3: 44.736 Mbits/sec
• International standards (based on the ITU hierarchy developed in
Europe) is somewhat different
– E1: 2.048 Mbits/sec supporting 30 voice channels at 64 kbits/sec each
with two channels for control.
SONET Digital Hierarchy
•
•
•
•
•
•
OC-1
OC-3
OC-12
OC-48
OC-192
OC-768
51.84 Mbps
155.52 Mbps
622.08 Mbps
2.48832 Gbps
9.95328 Gbps
 40 Gpbs
• Compare to the evolution of standard rates for Ethernet
–
–
–
–
–
10 Mbps
100 Mbps
1 Gbps
10 Gbps
40 Gbps
Wavelength Division Multiplexing (WDM)
• Combining multiple wavelengths over the same optical fiber
• For example 16 wavelengths, each at 2.5 Gbps can provide a rate
of about 40 Gbps. (OC-48 x 16)
• Using optical add drop multiplexers
• Optical switches
• A lightpath is a logical connection from a start node to an end
node going through intermediate nodes.
–
–
–
–
Same wavelength throughout
Reallocating of a wavelength at intermediate nodes (switching)
Protection paths
Wavelength switching capabilities
Parallel transfer of information (bits)
Parallel Wires
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8 bits, 16 bits, etc.
Serial Transfer (Bit Serial)
0 0 0 0 1 1 0 0 1 0 0 0 0
Data line
DEST
SOURCE
Control line / info
Transfer, for example, an octet a single bit at a time using a fixed time
interval for each bit. A clock signal helps determine when to sample the
bit.
Why use a serial line? As distances increase, multiple wire costs increase,
complexity of line drivers and receivers increase, etc.
Thus, to send a character, first serialize the bits, send over the line, and
then receive the bits, and convert back to character.
Communication Modes
• Simplex: Data is transmitted in one direction only
• Half duplex: Alternating exchange of data between two devices.
Need to switch between sender and receiver mode
• Full duplex: Data can be exchanged in both directions
simultaneously
Determining Received Bit Pattern
Determine
•
Start of each bit (or center of bit).
•
Start and end of the unit, a character, a byte, etc. Byte synchronization
•
Start and end of message unit or frame.
Bit or clock synchronization
Frame synchronization
Asynchronous Transmission
• Receiver and transmitter clocks are not synchronized. They are
independent. Receiver resynchronizes at the start of each unit,
say a character.
• To determine beginning of character, have a “start” bit
• To be sure about end of character, have a stop bit.
• Know how many bits are in a character, say 8.
• Clock frequency for sampling is usually 16 times bit rate.
Receiver attempts to sample in the center of the bit.
• Start bit is usually 1 bit long and stop bits are often 1, 1.5, or 2
bits
(idle)
start
1
0
0
1
1
1
1
0
stop
(idle)
Synchronous Transmission
• Sender and receiver clocks are synchronized (often through the
sending of a clock signal on a control line)
• Start and stop bits are not used. Characters can be sent one after
the other without the overhead of start and stop bits.
• Framing is still important.
1850 – Invention of telegraph by Morse
1874 - Invention of time-division multiplexing by Baudot
1876 - Invention of telephone by Bell
1899 - Invention of radio by Marconi
1928 – Development of sampling theory by Nyquist
1936 - Invention of pulse code modulation by Reeves
1948 – Development of channel capacity theory by Shannon
Invention of transistor by Bell Laboratories
1962 – First 1.544 Mb/s T1 cable carrier system by Bell System
Telestar, first communications satellite be Bell System
1965 – Early Bird, first geostationary communications satellite by INTELSAT
1966 – Low-loss optical fiber proposed by Kao
1980 – AT&T introduces fiber optic transmission at T3 data rate
1984 – Divestiture of the Bell System increases competition and accelerates introduction of digital
transmission in the United States
1988 – First transatlantic fiber optic cable (TAT-8) installed
1989 – First transpacific fiber optic cable (HAW-4/TPC-3) installed
1992 – Conversion of major U.S. networks to digital transmission completed
1990s- Deployment of digital transmission worldwide in support of the Synchronous Digital Hierarchy
and Integrated Services Digital Network.
Performance of Transmission System
Circuit length
Type of service
Reference
circuit
Analog vs.digital
Multiplex
hierarchy
Performance
objectives
Performance
objectives
Quality
Bit error
rate
Bit count
integrity
Availability
Jitter
Delay
Propagation
reliability
Equipment
reliability
Voice Transmission
For digital transmission applications, voice signal must first be converted to a digital
representation by the A/D process
-
PCM (Pulse Coded Modulation)
Different PCM
Delta modulation
Analog
input
Digital
output
Low-pass
filter
\´
Quantizer
Coder
Predictor
Block Diagram of Analog-to-Digital Converter
 Standard for voice digitization
• For single channel: PCM at a transmission rate of 64 kb/s (8 kHz sampling with 8 bit
quantization)
• For multi-channel:
North American standard: 24 voice channels at a total transmission rate of 1.544 Mb/s
European CEPT standard: 30 voice channels at a total transmission rate of 2.048 Mb/s
 Transmission quality of digitized voice channels
• A/D technique
• Number of tandem A/D and D/A conversions (accumulation of quantization noise)
• Characteristics of transmission impairment
Data Transmission
Interface
DTE
Data
terminal
equipment
data
timing
control
Interface
DCE
Data circuit
terminating
equipment
DCE
Communications
Network
Data circuit
terminating
equipment
Data Transmission Network Interfaces
data
timing
control
DTE
Data
terminal
equipment
 Timing Signals
Asynchronous transmission
Synchronous transmission
Timing
Timing
DTE
Data
DCE
Communication
Network
DCE
Data
DTE
(a) Codirectional timing
Timing
Timing
DTE
Data
DCE
Communication
Network
(a) Contradirectional timing
DCE
Data
DTE
Timing
Timing
DTE
•
•
•
DTE
Buffer Data
Data
Timing
Data
•
•
•
Timing DCE
Communication
Network
Buffer Data
(c) Buffered timing
Synchronous Data Transmission Configuration
 Interface standards
Application
End-to-end______________
Layer7_______________
Presentation
End-to-end______________
Session
End-to-end______________
Transport
End-to-end______________
Layer6_______________
Layer5_______________
Layer4_______________
Network
________________________
Layer3_______________
Data Link
________________________
Network
Layer2_______________
Physical
Layer1_______________
DTE
Communications
________________________
Equipment
DCE
Architecture of Open Systems Interconnection Model
Video Transmission
• Compression schemes are essential because of very large
amounts of information to be transmitted. (e.g.) 8-bit PCM
applied to 6-MHz TV signal results in 96 Mb/s
• Compression schemes
- Compression in space: transform coding, sub-band coding
- Compression in time: motion estimation/compensation
Failure Model
Failure : departure from specified performance
•
Failure rate ( ): average rate at which failures can be expected to occur
throughout useful life of equipment
•
Reliability (R(t)): probability that equipment will perform without failure for a
period of time t given by
R(t )  e
•
 t
Note that the above equation is based on the Poisson assumption on the failure
process.
Performance Objectives
•
Availability is defined as the probability or fraction of time that circuit continuity is
maintained and usable service is provided.
•
Quality is considered only when the circuit is available and the quality parameters
commonly used include bit error rate, timing slips, jitters, and delay.
•
Design objectives (DO) vs. Operation and Maintenance (O&M) standards of
performance.
DO=O&M + Margin
•
Mean Time Between Failure (MTBF): ratio of total operating time divided by the
number of failures in the same period.
MTBF 


0
t (e  t ) dt 
1

•
Mean Time To Repair (MTTR): average time to repair the failed equipment including
fault isolation, equipment replacement or repair, and test time, but excluding
administrative time for travel and locating spare modules of equipment.
•
Mean Time to Service Restoration (MSTR): average time to restore service including
repair and administrative time.
•
Availability (A): probability that equipment will be operable at any given point in time
or the fraction of time that the equipment performs the required function over a stated
period of time, which can be defined as follows:
A
MTBF
MTBF  MTSR
•
Unavailability (U): complement of availability :
•
For large MTBF and small MTSR (usually the case)
U 
MTSR
MTBF
U  1 A 
MTSR
MTBF  MTSR
•
Outage (O): condition whereby the user is deprived of service due to failure within the
communication system.
•
Mean Time Between Outages (MTBO): ratio of total operating time divided by the
number of outages in the same period.
 Redundant equipment
•
MTBO1: If the equipment consists of n identical units, each with the same MTBF, for
which at least r must be operable for the system to be operable, the first type of outage is
given by
MTBF( MTBF / MTTR) nr
MTBO1 
,
n(n  1)!/(r  1)!(n  r )!
for MTTR  MTBF
where the sensing of a failure and resulting switchover is itself to occur within failure.
 System availability
•
Series combination:
N
AN  II Ai ,
i 1
N
N   Ai ,
i 1
N
U N  1  II Ai .
i 1
M
•
Parallel combination:
AM  1  II U i .
i 1
•
•
M


 II 1  II U ij .
i 1 
i 1

N
Series parallel combination:
ANM
N


 1  II 1  II Aij .
i 1 
j 1

M
Parallel-series combination: AMN
1
N
2
Series Combination
1
2
N
Parallel Combination
A1
B1
N1
A2
B2
N2
AM
BM
NM
Series-Parallel Combination
A1
A2
AN
B1
B2
BN
M1
M2
MN
Parallel-Series Combination
Error Objectives
 Error parameters
•
Bit Error Rate (BER): ratio of erred bits to the total transmitted bits in a measurement
interval
•
Error-Free Second (EFS) or Error Seconds (ES): percentage or probability of onesecond measurement intervals that are error free (EFS) or in error (ES)
•
Error-Free Blocks (EFB): percentage or probability of data blocks that are error free.
 Relationships between error parameters
•
Basic assumption: statistically independent bit errors with an average probability
of error
p=BER, R = bit rate
•
EFS and BER:
%EFS 100(1-p)R
n k
    p (1  p ) n  k  1  (1  p ) n  np for np  1
k 1  k 
n
•
P (block error)
EFB  (1  p ) n
Probability of block error
1
Error second probability
Bit error probability
0
1
R(data rate)
Block length
Bit Count Integrity
 Definitions
•
•
Bit Count Integrity (BCI): preservation of precise number of bits (or characters
or frames) that are originated in a message or unit of time.
Slip: loses of BCI caused by a shift of a digital signal.
 Performance Impact of One Slip
Service
Encrypted Text
Video
Digital Data
Facsimile
Voice Band Data
Voice
Potential Impact
Encrypted key must be resent
Freeze frame for several seconds - Loud pop on audio
Deletion or repetition of data - Possible reframe
Deletion of 4-8 scan lines – Drop call
Transmission errors for 0.01 to 2 sec. – Drop call
Possible click
 CCTTT Performance Objectives for slip rate on 64-kb/s International
Connection (Rec. G.822)(13)
Performance
Classification
Unacceptable
Degraded
Acceptable
Mean Slip Rate
Thresholds
>30 slips in 1 hr
>5 slips in 24 hrs and
<30 slips in 1 hr
<5 slips in 24 hrs
Measurement
Average Period
1yr
1yr
Percentage of
Total Time
<0.1%
1yr
>98.9%
Jitter
 Definitions
•
•
Jitter: short-term variation of sampling instant from its intended position in time or
phase
Wander (or drift): long-term variation of sampling instant fro its intended position in
time or phrase
 Transmission impairment
•
By jitter:
* Degradation in system error rate performance
* Slips
* Distortion in the recovered analog signal
•
By wander:
* Buffer overflow (fill) or underflow (empty)
Sources of jitter
• Regenerative repeaters: imperfect clock recovery process
• Digital multiplexers: insertion/deletion of overhead of
stuffing bits
Sources of wander
• Oscillator instability
• Propagation delay variation